1. Field of the Invention
The present invention relates to a projecting optical system and a projection apparatus using the same and, more particularly, to a projecting optical system whose constituent lenses are appropriately designed so as to project, in an enlarged scale, an original image displayed on an image modulating element, for example, a liquid crystal panel (liquid crystal display element), onto a screen and which is suited to a liquid crystal projector while maintaining a good telecentric form and having a high optical performance.
2. Description of Related Art
Heretofore, there have been proposed various projecting optical systems and projection apparatus using the same, which each project in an enlarged scale an original image, such as an image on a film or on a liquid crystal light bulb, onto a screen.
For the projecting optical system, a wide variety of types of optical systems have been used. In the case of a projecting optical system using a color liquid crystal display as an original image to be projected, there is a demand for an elongated back focal distance because the space between the last lens surface and the liquid crystal display elements (back focal distance) must accommodate an optical member such as a reflecting mirror or a dichroic mirror. For this reason, the type in which a lens unit of negative refractive power leads, i.e., the retrofocus type, is used in most projecting optical systems for color liquid crystal display.
FIG. 16 is a schematic diagram showing the main parts of a projection apparatus using a reflection-type liquid crystal display device, as proposed in Japanese Laid-Open Patent Application No. Hei 6-265842. The projection apparatus shown in FIG. 16 includes, in order from the side of a screen 19, a projection lens 18 for projecting original images onto the screen 19, a stop 32, a plano-convex lens 33 having a convex surface facing the screen 19, and a cross dichroic prism 34 for three-color separation.
Images to be projected in different colors, for example, red, green and blue, are formed in liquid crystal panels 13, 14 and 15 of the reflection type, respectively. A reflecting mirror 31 occupies a half of the aperture opening of the stop 32 to introduce illumination light for illuminating the liquid crystal panels 13, 14 and 15.
Light emitted from a light source (incandescent lamp) 1 advances directly or after being reflected by a reflector 2 toward a mirror 20 and, then, is reflected by the mirror 20. The reflected light is condensed by a condenser lens 30 and, then, is reflected by the reflecting mirror 31 disposed in the aperture of the stop 32. The light reflected by the reflecting mirror 31 is guided to the plano-convex lens 33 and, then, is collimated by the plano-convex lens 33. The collimated light is separated by the cross dichroic prism 34 into light beams of three colors, i.e., red, green and blue. Then, the reflection-type liquid crystal panels 13, 14 and 15 are illuminated with the light beams of the three wavelength regions.
Light beams modulated by the reflection-type liquid crystal panels 13, 14 and 15 are combined in color by the cross dichroic prism 34. The combined light beams are condensed by the plano-convex lens 33, are transmitted through the aperture of the stop 32 and are guided to the projection lens 18. The projection lens 18 projects onto the screen 19 the color-combined light beams from the liquid crystal panels 13, 14 and 15.
In general, a projection apparatus using the liquid crystal panels as original images to be projected is provided with a color separating means (cross dichroic prism, dichroic mirrors, or the like) between the projecting optical system (projection lens) and the liquid crystal panels. For this reason, the projecting optical system needs a long back focal distance.
Meanwhile, in order to prevent the unevenness of colors on the screen from occurring, it is necessary to make constant the angles of incidence on the dichroic surfaces of the color separation prism at any point in the original images to be projected. Since the liquid crystal panel has the angle response characteristic, it is also required to make small the angles of incidence of the off-axial rays on the display surfaces. For example, it is necessary to make the off-axial rays incident on the display surface almost at right angles.
Because of the existence of such requirements, the use of the liquid crystal panels as the original images to be projected leads to the necessity of constructing the optical system in the form of the retrofocus type to increase the back focal distance and in the form of an image-side telecentric system.
However, with the projecting optical system constructed in the form of the retrofocus type, the lens system becomes asymmetric, so that various aberrations increase greatly, thereby making a good optical performance difficult to obtain. Also, the use of the image-side telecentric system causes an increase of the number of lens elements, producing another problem in that the entire lens system becomes complex in structure and increases in size.
Also, the projection apparatus using the cross dichroic prism as the color separating means has the following problems:
(i) As the number of layers in the coating of the cross dichroic prism is 20 or thereabout at the lowest, the spectral characteristic differs greatly from item to item. In terms of manufacturing, therefore, the spectral characteristic is difficult to keep at a certain level. PA1 (ii) The prism elements constituting the cross dichroic prism must be assembled at a high angular accuracy. Otherwise, the projected image would be split double. PA1 (iii) On one hand, the cross dichroic prism lies just before the liquid crystal panels. To illuminate the liquid crystal panels, on the other hand, the light beams to be used have to be collimated. The plano-convex lens is, therefore, positioned a long distance away from the liquid crystal panels. Therefore, the light beam passing through the plano-convex lens increases in diameter, causing correction of astigmatism and coma to become difficult.